diff --git a/secop_psi/phytron.py b/secop_psi/phytron.py
index 14f596e..614814d 100644
--- a/secop_psi/phytron.py
+++ b/secop_psi/phytron.py
@@ -40,7 +40,8 @@ class PhytronIO(StringIO):
                 _, _, reply = super().communicate('\x02' + command).partition('\x02')
                 if reply[0] == '\x06':  # ACK
                     break
-                raise CommunicationFailedError('missing ACK %r' % reply)
+                raise CommunicationFailedError('missing ACK %r (cmd: %r)'
+                                               % (reply, command))
             except Exception as e:
                 if ntry == 1:
                     raise
@@ -87,10 +88,19 @@ class Motor(PersistentMixin, HasIO, Drivable):
         self.loadParameters()
 
     def get(self, cmd):
-        return self.communicate('\x02%x%s%s' % (self.address, self.axis, cmd))
+        return self.communicate('%x%s%s' % (self.address, self.axis, cmd))
 
     def set(self, cmd, value):
-        self.communicate('\x02%x%s%s%g' % (self.address, self.axis, cmd, value))
+        # make sure e format is not used, max 8 characters
+        strvalue = '%.6g' % value
+        if 'e' in strvalue:
+            if abs(value) <= 1:  # very small number
+                strvalue = '%.7f' % value
+            elif abs(value) < 99999999:  # big number
+                strvalue = '%.0f' % value
+            else:
+                raise ValueError('number (%g) must not have more than 8 digits' % value)
+        self.communicate('%x%s%s%s' % (self.address, self.axis, cmd, strvalue))
 
     def set_get(self, cmd, value, query):
         self.set(cmd, value)
@@ -103,7 +113,7 @@ class Motor(PersistentMixin, HasIO, Drivable):
             enc = self.read_encoder()
         else:
             enc = pos
-        status = self.communicate('\x02%xSE' % self.address)
+        status = self.communicate('%xSE' % self.address)
         status = status[0:4] if self.axis == 'X' else status[4:8]
         self.log.debug('run %s enc %s end %s', status[1], status[2], status[3])
         status = self.get('=H')